In its most elemental way, a source of electrical power is typically a battery which may comprise one or more battery cells. Each cell typically comprises encapsulized electrolyte and positive and negative electrodes. During cell operation, electrons move through the solid electrode material, to the electrolyte/electrode interface. There, a faradaic (charge-transfer) reaction occurs, which transfers the charge from electrons to electrolyte species. Ions then flow through the electrolyte to the opposite electrode, where another faradaic reaction takes place, liberating electrons into the solid electrode material. Electrons then flow from the electrode to the external load connected to the battery.
Because of a number of fundamental deficiencies, including but not limited to ionic resistance and electronic resistance within the cell, prior battery technologies have proven to be unsatisfactory for high discharge and high recharge power requirements including those imposed in the operation of an all-electric or hybrid electric vehicle. Electric vehicles typically take the form of parallel-configured and series-configured vehicles.
Parallel-configurated electric vehicles require a battery pack which is smaller in size, and yet can be both discharged and recharged at rates comparable to those specified for the series-configured hybrid. No battery presently available can approach the power requirements (especially charging power) for the parallel-configured hybrid vehicle.
The limitations in power are not necessarily due to the fundamental electrochemistry of the battery systems, but instead are often due to certain design constraints of the batteries, particularly the electrodes. Among the design constraints of prior battery packs for electric vehicles are:
excessive solid-phase resistance to electronic current flow;
excessive electrolyte-phase resistance to flow of ionic current within the electrode; and
excessive kinetic resistance in the electrode, caused at least partially by the nature of electrode surface area.
Prior spiral lead acid batteries often perform better at high rates than prismatic (parallel-plate) batteries. Still, the performance of prior spirally-wound lead-acid cells is not adequate for many load-levelling applications requiring high rate charging and discharging, such as hybrid electric vehicles.
Prior silver-zinc batteries each consists of a zinc electrode (the negative), a silver-oxide electrode (the positive), and an alkaline electrolyte. Each electrode is supported by some conductive grid. Numerous materials have been used, and zinc metal (for the negative) and silver metal (for the positive) are common. In the fully-charged state, the negative consists of zinc metal (usually porous), and the positive of AgO (Ag2O is also used). The discharge reactions involveZn−>ZnOAgO−>Ag2O−>Ag.
The open circuit potential of the fully-charged prior cell is around 1.83 V, depending on the electrolyte (KOH) concentration. This battery is the highest-energy density battery using an aqueous electrolyte. It is also capable of high power density. Cycle life is short due to the solubility of both zinc and silver, and the aggressive action of silver on separator materials. The cost is obviously high, such that its application is usually limited to military and aerospace purposes, where energy density is of primary importance, and cost is not.
Spiral wound lead acid batteries are known wherein electrodes are made by applying appropriate pastes to a lead or lead alloy grid, and curing the paste to form the electrode active materials. Pastes are composed of lead oxides, sulfuric acid, and other components. The compositions will vary depending on the vendor. Cells are made by placing separators between the pasted electrodes and rolling the electrodes and separators into a coil. The separator material is usually compressed during cell winding. Consequently, the space between electrodes is conveniently made small, which provides somewhat lower internal cell resistance.
Spaced rectangular flat plate electrodes in lead acid are also known, which are made by the so-called Planté process. This process originally involved the simple electrochemical oxidation of lead metal in sulfuric acid. The resultant capacity of the electrodes was low, but gradually increased as the cells were charged and discharged. The resulting electrodes proved very durable, but suffered from low specific capacity (A·hr/cm2).
Later improvements allowed for increased capacity. The first was the addition of lead-solubilizing agents2 (such as KClO4, KClO3, HCl, HNO3, and H2C2H3O2) to the sulfuric acid formation solution. The use of these agents resulted in more rapid and deeper penetration of the corrosion layer on the flat electrode plates during oxidation leading to higher specific capacity. The second was the mechanical working of the lead to increase surface area, such as by creating lead ridges on the flat electrode plates. This further increased the electrode capacity.
During the early part of this century, Planté electrodes were gradually replaced by pasted electrodes for most energy storage applications, due to the higher specific capacity, and despite the shorter life expectancy. Planté electrodes are still used commercially in applications where long life is paramount, especially stationary applications (such as stand-by power). A number of manufacturers still produce Planté batteries in the 100-2000 A·hr range and the U.S. Department of Defense uses Planté batteries for some stand-by power supply applications where long life is needed. The principal advantage of Planté batteries is in their stability. It is not uncommon for Planté batteries to have a useful life of decades.
The processing of Planté flat plate batteries are typically costly and time intensive compared to pasted electrode batteries. Flushing of potassium perchlorate adds significantly to the time and cost requirements.